The present invention relates to alternating current (xe2x80x9cACxe2x80x9d) power supplies, in general. More particularly, the present invention relates to the supply of AC power from a cable television (xe2x80x9cCATVxe2x80x9d) system into the home, and detecting a resistive load connected to the AC power source.
Known power delivery systems use ground fault interrupters (xe2x80x9cGFIsxe2x80x9d) that turn off electrical power when they sense an unbalance in the current flow between the hot and neutral wires that deliver AC electrical power. GFIs may be able to sense if a person or animal is touching the hot and neutral wires, thereby completing the circuit. GFIs, however, are not applicable in situations where the hot and neutral wires do not carry equal currents, such as CATV cables, because another return, namely ground, exists between the neutral-connection-to-the-load and the source of the AC power.
Power to loads can be turned on and off by switching circuit systems such as field effect transistors (xe2x80x9cFETsxe2x80x9d), silicon controlled rectifiers (xe2x80x9cSCRsxe2x80x9d), triacs, bipolar transistors, or relays. When FETs are used for switching circuits, complex level-shifting circuits or optical isolators are needed to properly drive the gate terminals of the FETs. FETs, however, are desirable for switching applications because switching AC power on and off using semiconductor junction-based devices, such as SCRs, triacs, bipolar transistors, generates electromagnetic interference and cross-over distortion, is less efficient and requires more drive power than FETs. Relays are slower than FETs and require increased drive power.
Known switching circuits use two FETs in series with their sources connected together. It may be difficult, however, to properly control the gates of the two FETs by biasing them with the proper voltages. This problem occurs because the FETs contain intrinsic diodes. During an off condition, the AC voltage passing through the intrinsic diodes causes the common source connections to swing widely in voltage, which can damage the gate drive circuits.
The present invention is directed to a method and a device to determine when a resistive load is connected to a source of AC power and shuts off power when a resistive load is detected. The device includes a signal path and an AC power path. The device also includes a microcontroller connected to a sensing circuit for determining an AC current level in the AC power path. The microcontroller indicates an off condition according to the current level. The device also includes a power switch for turning off the AC power when instructed by the microcontroller.